Are we overcalling sacroiliitis on MRI? Differential diagnosis that every rheumatologist should know - Part I

Diagnosing early spondyloarthritis remains a challenge in routine practice, especially in its axial (SpA) form. Magnetic resonance imaging (MRI) is capable of detecting early bone marrow edema (BME) in the sacroiliac joints (SIJs), a key criterion for the diagnosis of active SpA according to the "imaging arm" of the Assessment of Spondyloarthritis International Society (ASAS) classification. However, despite MRI having superior reliability compared to radiographs and being recognized as a crucial imaging biomarker of SpA, it has several limitations, including its limited specificity and sensitivity. Based on the established literature, there is currently an "overcall" of sacroiliitis on MRIs. In this setting, differential diagnoses and their imaging features come into play. In this two-part article, we will review both the imaging features that constitute a "positive" MRI in SpA and the most common differential diagnoses. In order to understand the pathophysiology of sacroiliitis and the spectrum of developing lesions, one needs to be familiar with the complex SIJs anatomy, both on radiographs and on cross-sectional imaging studies (particularly MRI). As such, in the first part of this article, we provide a brief background on anatomy and different imaging modalities used in this clinical setting and we review the imaging criteria for a "positive" MRI study of sacroiliitis in adults (imaging arm of the ASAS classification).